The present invention relates to an apparatus for driving an objective lens for correcting a relative positional error, i.e., a tracking error between one or more information tracks recorded spirally or concentrically on a disc-shaped record medium and a light spot projected onto the record medium by means of the objective lens.
In an apparatus for reproducing or picking-up an information from the above mentioned record medium, the record medium is usually called as a video disc in which encoded video and audio signals are recorded as optical information such as an optical transmitting, reflection, phase property. While the video disc is rotated at a high speed such as thirty revolutions per second, i.e. 1,800 rpm, a laser beam emitted from a laser light source such as a helium-neon gas laser is focussed on the tracks of the disc as a light spot and the optical information is read out. One of important properties of such a record medium is a very high density of recorded information and thus a width of the information track is very narrow and a space between successive tracks is also very narrow. In a typical video disc described in, for instance, Philips Technical Review, Vol. 33, 1973, No. 7, a pitch of the tracks amounts only to 2 .mu.m. Therefore the diameter of light spot should be correspondingly small such as 1 to 2 .mu.m. In order to pick-up correctly the recorded information from such tracks having very narrow width and pitch a mutual positional error between the light spot and tracks, i.e., a tracking error should be reduced as little as possible. In U.S. Pat. Nos. 3,909,608 and 3,882,317, it has been proposed to effect a so-called tracking control in which a mutual displacement of the light spot and track is photoelectrically detected to produce a tracking error signal and the light spot is moved or shifted in a direction perpendicular to the track in accordance with the detected tracking error signal. In the U.S. Pat. No. 3,882,317 as a tracking mechanism use is made of an oscillating mirror inserted in an optical path from a laser light source to an objective lens, and the mirror is rotated in accordance with the detected tracking error signal. However such a tracking mechanism could not attain a sufficiently high accuracy and is liable to be large in size. Further the known tracking mechanism is rather expensive.
In order to avoid the above mentioned drawback of the known tracking mechanism it has been further proposed that the objective lens on its holder is supported by a resiliently supporting member comprising springs, e.g. leaf springs, and the objective lens is moved in a direction perpendicular to the optical axis of objective lens as well as to the tangential direction of the information track by means of an electromechanical transducer in accordance with the tracking error signal. Hereinafter the direction in which the objective lens is moved for effecting the tracking control is termed as a tracking direction. As the electromechanical transducer use may be made of electromagnet, voice coil, piezoelectric element, etc. ln order to obtain a good response property for tracking the assembly should be made small in size and light in weight. In an actual reading-out apparatus in addition to the tracking error a so-called focussing error is produced in which the light spot could not be correctly focussed on the information track. For correcting the focussing error there should be provided with a focussing mechanism for moving or displacing the objective lens in a direction of its optical axis. When the tracking mechanism is installed on the focussing mechanism, the tracking mechanism should be smaller in size and lighter in weight for attaining an accurate focussing correction. In the tracking mechanism with the electromagnet a sufficient force necessary for moving the objective lens can be produced and the mechanism can be constructed small and light in a relatively simple manner. However, an accurate tracking correction could not be effected, because a relation between an amount of electric current passing through a coil of the electromagnet and an amount of displacement of the objective lens is not linear. In the tracking mechanism comprising a voice coil, a miniaturization could hardly be attained. Further the tracking mechanism with the piezoelectric elements could not produce a sufficiently large driving force for driving the objective lens.